1. Field of the Invention
The present invention relates to coal processing and handling. More particularly, this invention relates to deactivation or passivation of coal or solid carbonaceous fuels to reduce the tendency of the material to spontaneously combust.
2. Description of Related Art
Solid carbonaceous materials, in particular solid carbon-based fuels, may autoignite or spontaneously combust under the proper conditions. Carbonaceous material may include coal, low-rank coal, dried coal, peat, char, or other porous solid fuel. For example, certain coals, such as sub-bituminous, lignite and brown coal, subsequent to mining can spontaneously combust due to chemical reactions between the coal, moisture and oxygen present in the air. This reaction can occur due to water combining with other components in the coal to generate a sufficient amount of heat to raise the temperature of the coal to the ignition point. Further, materials present in the coal may oxidize upon exposure to air, which in turn generates a sufficient amount of heat for the coal to reach ignition temperature. The components being oxidized within the coal may be non-carbonaceous matter or unsaturated carbon compounds within the coal. Certain coals, which are normally stable with respect to autoignition after mining, may be brought into proper conditions for autoignition after subsequent processing. For example, many low-rank coals contain significant amounts of free moisture. After drying to remove excess moisture these coals present a significant autoignition hazard.
Low-rank coals, such as sub-bituminous coal or lignite may contain more than about 10% moisture and typically 15-50 weight percent moisture. Some low-rank coals may contain as much as 60 weight percent moisture. Such wet low-rank coals cannot be shipped economically over great distances due to the cost of transporting a significant fraction of unusable material in the form of water. Further, these low-rank coals cannot be burned efficiently due to the energy required to vaporize the water. Due to the lowered heating value and high cost of shipping unusable material, it is advantageous to remove all or part of the water from the low-rank coals prior to shipment and/or storage. However, drying such fuels usually leads to activation of the low-rank coals or chars. The reactive coals or chars may be hazardous due to the potential for damage to property or life due to the reaction of the coal or char with atmospheric oxygen and moisture and consequential heating of the coal, which makes it subject to spontaneous ignition during either shipment or storage.
Indicators of the propensity of coals or chars to spontaneously combust include the uptake of oxygen as measured in terms of torr of oxygen per gram of material. Methods for testing this indicator are listed in U.S. Bureau of Mines "Report of Investigation 9330" by Miron, Smith, and Lazzara. The terms "oxygen uptake" and "oxygen demand" refer to the test methods of the "Report of Investigation 9330" or related test methods when used in this document.
In the past, wet low-rank coals such as those from the western United States have been dried by methods such as, but not limited to, thermal drying using process heat, waste heat, microwaves, pressurized water, steam, hot oil, molten metals, and other supplies of high temperatures. The heated coals release the free moisture trapped in the pores, water molecules associated with hydrated molecules or associated in other ways with the coal, producing dried coals or chars. Other methods of drying may include mechanical drying (such as centrifugal separation), the use of dry gases, or the use of desiccants or absorbents. Once dried, coals or chars can become more active and are known to spontaneously combust.
One approach to reduce the potential for the spontaneous combustion of the carbonaceous material, such as dried low-rank active coal or char (those susceptible to spontaneous combustion), is to seal the exterior surface of the char by using oils, polymers, waxes or other materials to coat the surface of the coal. Examples of such coating processes are U.S. Patent Numbers 3,985,516 and 3,985,517 to Johnson, which disclose heating and intimate mixing of coal with heavy oils to coat the particles. Such coating procedures are rather effective in preventing reabsorption of moisture by the char, however, such coatings are expensive due to the cost of the hydrocarbon materials added and thus are unattractive. It would be advantageous to dry wet coals and process them in such a manner that the dried coal or char particles are made less reactive after moisture removal, so as to prevent the reaction of the carbonaceous material with oxygen without the need for externally supplied coating materials. An alternate method to reduce spontaneous combustion is the prolonged exposure of the coal to air. Another method includes the use of oxidizing agents sprayed on coal.
Another method to treat the carbonaceous material is the use of high-temperature water under pressure. The coatings perform their work by covering the pores and limiting the access of active components of the air to active sites in the material (dried coal in this instance). U.S. Pat. No. 1,632,829 to Fleissner discloses a process for drying wet coal by steam heating it using a procedure wherein steam provided above the coal is maintained at high partial pressure such that moisture will not escape during coal heat up, then reducing the steam pressure to permit the escape of moisture and rapid drying of the coal. Also, U.S. Patent Number 4,052,169 to Koppelman discloses a process for upgrading lignitic coal, comprising heating it in an autoclave at about 750.degree. F. temperature and 1000 psig or more pressure to effect thermal restructuring, followed by cooling and depositing condensible organic material on the lignite to provide a stabilization of the upgraded product and render it non-hygroscopic and more resistant to weathering and oxidation during shipment and storage. The use of high temperature water is reported to drive off carboxylic acid groups and thereby remove those sites from future activity with the active components of the fluid.